The Baldur's Gate: Enhanced Edition Website announces Overhaul Games' upcoming Baldur’s Gate overhauls will be coming to OS X this summer (thanks The Patches Scrolls). They say the Mac version will "include a variety of Mac OS X specific features, including widescreen and iCloud support." No, we didn't know widescreen support was an OS X specific feature either. Technically a PC edition of the game has not been announced, but it is mentioned in this tweet from Trent Oster revealing they plan to support cross-platform multiplayer play:

Yes, it will be compatible multi-player cross-platform. We had a Mac, iPad and PC joined into a game together.